In a wireless communication system, a central base station communicates with a plurality of remote terminals, such as cellular mobile phones. Frequency-Division Multiple Access (FDMA) and Time-Division Multiple Access (TDMA) are the traditional multiple access schemes to provide simultaneous services to a number of terminals. The basic idea behind FDMA and TDMA technics is to slice the available resource into multiple frequency or time slots, respectively, so that multiple terminals can be accomodated without causing interference.
Contrasting these schemes with separate signals in frequency or time domains, Code-Division Multiple Access (CDMA) allows multiple users to share a common frequency and time channel by using coded modulation.
More precisely, as it is well-known by the man skilled in the art, a scrambling code which is a long pseudo noise code sequence, is associated with each base station and permits to distinguish the base stations from each other. Further, an orthogonal code, known by the man skilled in the art under the denomination of OVSF code, is allocated to each remote terminal (such as cellular mobile phone). All these OVSF codes are orthogonal with each other, which permits to distinguish a remote terminal from another.
Before emitting a signal on the wireless transmission channel towards a remote terminal, the signal has been scrambled and spread by the base station using the scrambling code of the base station and the OVSF code of the remote terminal.
Because of possible reflections of the initial transmitted signal on obstacles between the base station and the remote terminal, the wireless transmission channel is in fact a multipath transmission channel. As a result, the signal which is received by remote terminal includes different time shifted versions of the initial transmitted signal which are the results of the multipath transmission characteristics of the mobile radio channel. Each path introduces a different time delay.
Among the CDMA systems, the CDMA-FDD systems use a different frequency for emission and for reception (FDD: Frequency Division Duplex), whereas the CDMA-TDD systems use a common frequency for emission and reception, but different time domains for emission and reception (TDD: Time Division Duplex).
The main problem arising from the use of CDMA is the Multiple Access Interference (MAI) from the users in the cell and the Inter Cell Interference (ICI) coming from other cells.
In recent years, multiuser detection has gained significant notoriety as a potential advanced technology for the next generation of CDMA systems. The poor code cross-correlation properties induced by the short spreading lengths in WCDMA/TDD) lead to severe degradations when several users are transmitting simultaneously, and the conventional correlation receiver appears to be limited. To overcome this major drawback, several advanced receiver structures have been proposed.
Unlike the conventional receiver, which treats multiple access interference (MAI) as if it were Additive White Gaussian Noise, multiuser receivers treat MAI as additional information to aid in detection. Interference cancellation (IC) is one of several multiuser detection (MUD) methods to suppress the effects from the MAI and consequently improve the resulting performance. This in return will increase the capacity of the communication system.
Interference cancellation at the mobile terminal is beneficial to perform at high data rates that will be supported by multicode transmission. Because of multipath propagation, the mobile will experience multicode interference, from itself as well as from the other users. Applying a MUD technique to the downlink is extremely important since the system capacity is limited at the downlink. This is further enhanced by the higher traffic requirements in the downlink, the possibility of soft handoff, and the possibility of having antenna diversity.
Multiuser detection is probably one of the best reception techniques) as it removes efficiently the multiple access interference. Among this method, the so-called MMSE (Minimum Mean Square Error) joint detection, well-known by the man skilled in the art, can be cited.
However, the major drawback of these conventional multiuser detection methods lies in its high computational complexity involved by huge matrix inversions. This extra complexity is not desired at the terminal units, which possess limited battery life and processing capabilities. Furthermore, such conventional methods assume the knowledge of the spreading codes of the interfering users. However, this information is not always available.